This invention relates generally to tuners, and more specifically to a system and method for a tuner which uses a single conversion to reduce a radio frequency input to a standard intermediate frequency signal.
Television tuners generally convert a radio frequency (xe2x80x9cRFxe2x80x9d) input into a standard intermediate frequency (xe2x80x9cIFxe2x80x9d) signal in preparation for further processing of the signal. Standard television sets typically use single conversion tuners to perform the conversion, while higher performance systems, such as set top boxes, typically use dual conversion tuners to perform the conversion. In the prior art, dual conversion tuners generally provide higher performance, but use more components and are more expensive than single conversion tuners. Although single conversion tuners generally provide lower performance than dual conversion tuners, single conversion tuners are desirable in that they generally use less parts and are less expensive. For example, a single conversion tuner uses only a single phase lock loop for providing a single local oscillator (xe2x80x9cLOxe2x80x9d) reference signal, as opposed to a dual conversion tuner, which requires two LOs. As another example, a single conversion tuner uses only a single IF filter and mixer for the conversion, whereas a dual conversion tuner requires two IF filters and two mixers for the conversion.
Prior art single conversion tuners are generally designed to process a narrow range of frequencies at any one time. This is accomplished through the use of a tracking filter on the front end of the tuner. As the receiver is tuned across the frequency band during a channel change, the tracking filter is tuned to allow only a few channels to pass into the tuner. As a result, the tuner circuit has to provide good response characteristics for only a few channels at a time, instead of over substantially the entire bandwidth. For example, in a cable television system the tuner would allow only a few channels to enter the receiver front end, instead of the full 100 or more channels in the total cable set. The cable channels may be at full strength of about 15 dBmV. The tracking filter beneficially reduces the dynamic range required in the front end of a conventional receiver.
There are several problems, however, associated with using a tracking filter in prior art single conversion tuners. Sometimes the tracking filter is located after an input low noise amplifier (xe2x80x9cLNAxe2x80x9d), which controls the input signal level, but more typically the tracking filter is divided into two sections, one part preceding the LNA and a second part following the LNA. The tracking filter generally must track the input frequency as the tuner is being tuned, and it is difficult to maintain good flatness, bandpass and signal rejection characteristics across the entire band.
In addition, in a single conversion tuner the LO is generally in-band, in that it is inside the frequency range of the overall number of channels received by the tuner. For example, if the tuner is tuned to channel 2 at 55.25 MHZ, the LO frequency is 45.75 MHZ above that, and the image frequency is located 45.75 MHZ above the LO. Therefore any undesired input which is in-band around that frequency (within about 6 MHZ) will pass through and be put on top of the desired signal. Therefore a tracking filter generally requires the use of a notch filter for suppressing the image frequency. This bandpass and rejection network which tracks the input and rejects the image frequency is undesirable because of its limited performance. In contrast, dual conversion tuners generally do not require the tracking filter or the notch filter.
A further disadvantage of having an in-band LO is that the oscillator frequency may leak out of the tuner into the broadcast medium. This may be especially problematic in a cable system, where the leaked LO signal may corrupt another channel where the LO is present.
Furthermore, the bandpass and rejection network generally requires relatively high voltage varactors to tune the network. Single conversion tuners thus typically require about 28 or 32 volts for the proper tuning of the bandpass and rejection network in the front end. This high voltage supply requirement is another undesirable feature of prior art single conversion tuners. Quite often, the tuning range of the varactors requires dividing the RF input spectrum into multiple bands. Typically, the input tracking filter is divided into three sections, as described in U.S. Pat. No. 4,598,425 entitled All-channel Television Tuning System, which patent is incorporated herein by reference.
There is therefore a need for a tuner with the benefits of a single conversion tuner, e.g., lower cost, along with the higher performance of a dual conversion tuner.
These and other objects, features and technical advantages are achieved by a system and method for a single conversion tuner which generally does not require a front end tracking filter. Through the use of broadband input LNA and mixer circuits, substantially the entire input signal bandwidth may be processed in the tuner. The broadband input LNA may pass the signal (together with the image) through to the mixers, which may split the signal into in-phase (xe2x80x9cIxe2x80x9d) and quadrature-phase (xe2x80x9cQxe2x80x9d) terms. Then the in-phase term may be shifted by plus 45 degrees and the quadrature-phase term may be shifted by minus 45 degrees. When the terms are subsequently summed together, the relative phase shifts cause the desired signal components to add together and the undesired image components to subtract from each other. In this way the image signal is suppressed, thus generally eliminating the need for a notch filter.
Because the signal is split and each portion travels through a separate circuit path, differences in the circuit paths, such as unmatched circuit components, may introduce errors, such as phase errors, when the signal components are subsequently combined. Therefore an error correction feedback loop may be implemented to compensate for these errors. The feedback loop may use a test signal to monitor the phase error between the in-phase and quadrature circuit paths, and may correct for the phase error by shifting the phase of the LO signal sent to one or both of the broadband mixers. The feedback loop thus generally corrects for any phase errors introduced by the image rejection circuitry.
A single conversion tuner in accordance with the present invention comprises image rejection circuitry comprising separate I and Q signal paths, wherein the I and Q signals are phase shifted and summed to substantially cancel an image channel from the signals, and phase error correction circuitry for measuring a phase error between the I and Q signal paths and adjusting the relative phase of the I and Q signal paths to substantially remove the phase error. The tuner may further comprise an injection test signal comprising first and second test tones, the first test tone having a frequency slightly lower and out of band from the image channel, and the second test tone having a frequency slightly higher and out of band from the image channel, wherein error measurements generated by the first and second test tones are averaged together to correct the phase error.
A single conversion method of converting a received RF signal into an IF output signal in accordance with the present invention comprises splitting the received RF signal into separate I and Q signal components, phase shifting the I and Q signal components, summing the I and Q signal components to generate the IF output signal, wherein an image channel is substantially canceled from the IF output signal, measuring a phase error between the I and Q signal components; and adjusting the relative phase of the I and Q signal components to substantially remove the phase error. The method may further comprise an injection test signal comprising first and second test tones, the first test tone having a frequency slightly lower and out of band from the image channel, and the second test tone having a frequency slightly higher and out of band from the image channel, wherein error measurements generated by the first and second test tones are averaged together to correct the phase error.
A technical advantage of one aspect of the present invention is that a single conversion tuner may be implemented with performance characteristics similar to those of a dual conversion filter.
Another technical advantage of one aspect of the present invention is that a tuner input signal may be split into its I and Q components, shifted in phase, and recombined to enforce the desired signal while suppressing the image signal.
Another technical advantage of one aspect of the present invention is that a phase error correction feedback loop substantially eliminates any phase error differences introduced by the separate I and Q signal paths.
Another technical advantage of one aspect of the present invention is that a front end tracking filter for limiting the input signal bandwidth is not required. Another technical advantage of one aspect of the present invention is that a notch filter for removing the image signal is not required.